This has been known to biologists for ages. In The Red Queen, I challenged readers to explain
why bodies should be designed that way: why set up an immune system
in such a way that it gets hindered by normal hormonal action? I
still find it baffling. Over the years readers took up my challenge
and wrote to me. They still do. Their answers nearly always boil
down to a version of this: to weed out weedy males. That is to say,
if males cannot both keep their testosterone levels up
and resist disease they don't deserve to contribute to posterity's
genes.

Trouble is, like all group selectionist arguments, it's
vulnerable to the evolutionary free rider. Along comes a mutant
animal that breaks the link between testosterone and illness and
hey presto it can breed away to its gonads' content, propagating
its subprime genes as if they were triple A.

Then there's the old 'constraint' argument - that the system's
built that way because it's the only way mother Nature knows how to
build it. Not very satisfying. Mutation finds a way round most
obstacles.

In the new study, Olivier Restiff says it's all about
reinfection. If males, which are the 'live hard die young' sex, are
only going to get reinfected quickly, then there's no point wasting
energy clearing previous infections. Divert the energy into
fighting instead.